Tuesday, July 24, 2012

I am watching an area of severe weather potential across the Great Lakes region, particularly over parts of the Northeast into the upper Western Great Lakes. Also of concern is the threat of overnight thunderstorms across the darker red region.

In the early hours of tomorrow, the 18z NAM model is projecting a cluster of showers and thunderstorms to develop across northern Minnesota and shift eastward in the midst of little instability but modest lifting. Of particular concern to this potential cluster will be how it reacts to what is forecast to be a strong cap over the region. It is a given that the lower level jet stream is nocturnal, and this is likely what will enhance storms to form over the area and keep them going in what would otherwise be an unfavorable environment for thunderstorms.

At this point, we are approaching late morning and the mesoscale convective system continues to show signs of continuing to move across the upper Midwest region, at this point now into far northern Wisconsin and the northern portion of Michigan. The NAM is progging the lower level jet stream to be in the 50-55 knot vicinity, enough to support strong to severe thunderstorms, even in the nighttime or late morning hours, when instability is traditionally not strong. I feel that the Lower Great Lakes may have to be on watch in this time as well, as the NAM is indicating a good 25-35 knot streak in the lower level jet may come to a point in the region.

Moving on into the wee hours of Thursday morning, the MCS makes an attempt to enter the Northeast, now once again holding its strength to the tune of possibly 60 knots of wind in the lower level jet stream. 3 hour precip forecasts, as shown above, are not as significant as they were on Wednesday morning's forecast, but remain significant enough to pose the question of severe weather potential. Notice the hints of blue in the western portions of the Lower Great Lakes. Further analysis of the 700mb winds does show over 30 knots of winds over that region, which would support some showers and storms, but not exactly on a severe level.

Here is the 1 hour precipitation forecast off the 21z Rapid Refresh model. It shows the bulk of the precipitation placed north of where the NAM is showing. This, in turn, would lead to a severe weather risk displaced farther to the north, should the low pressure system responsible make a move for this solution. This image is valid at about 9:00 AM CDT tomorrow and is long range for this short range model. However, it is indeed worth watching and will need closer monitoring into the morning hours tomorrow.

The derecho that has been in the realm of possibility appears to be on its way to forming, with the mesoscale convective system (MCS) now moving into the Ohio Valley and Mid Atlantic as shown below on radar. Derecho composite values, also shown below, illustrate the potential for a derecho to form in an area. As you can see, those values are elevated where the storms exist.

There has been a lot of interest over the potential for a derecho in the Ohio Valley and Mid Atlantic today, and after looking at the models, I believe that it is a possibility. However, the models have come in weak this morning. They still express a derecho-looking bowing segment to charge into the Ohio Valley and Mid Atlantic, but it looks weaker on this morning's runs than last night's models.
To the left, we see Northern Indiana's NWS office's WRF model. At roughly 1:00 PM CDT, this WRF is forecasting a bowing segment to hit the states of Kentucky and West Virginia. The 0z NAM, which had been portraying at least a moderate derecho, now appears to be seeing more of a cluster of showers and storms that could end up resembling that of a bowing segment. The general model consensus seems to be on the dim side as for derecho potential. But is that true after all?

The latest severe weather outlook for wind shows a 30% area for damaging winds in the areas being watched for the possible derecho. Considering the above image was for 1:00 PM CDT and the storms would likely strengthen as time goes on, personally, I do find it somewhat difficult to agree with the models. With the assistance of this Ring of Fire around a high pressure system, the upper winds aloft would seem to support more of a derecho solution panning out, especially with the lower level jet stream going through that area.

All in all, I personally see a derecho still on the table, but the models seem to be on the iffy side with it. We will see what happens this afternoon.

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